Movies and OVAs are non-canon so you don't have to watch them to keep up with the plot.
AO (anime original) episodes usually don't contribute to the plot either.
However, I would recommend watching OVA 9 just because it's such a good rendition of what could happen if Conan never finds the antidote.
As for the episodes, I would suggest you visit this website to keep up with the plot:
http://xerbladex.blogspot.com/p/detective-conan-important-episode-list.html
Also, although the older episodes seem to be of lesser quality than the newer ones, I highly recommend you catch up on those if you find the time because they really do a great job capturing the essence of what Detective Conan really is. The newer episodes tend to be lacking in that aspect.
If you don't want to be spoiled, don't go on the wiki too often because most of the pages contain major spoilers.
I think that's about it...
but if you have any other questions, there are a lot of friendly people around here who'll be happy to lend a hand.
Happy watching!